A number of medical ailments are treated or treatable through the application of electrical stimulation to an afflicted portion of a patient's body. Neurons, muscle and tissue cells are all forms of biological tissue capable of carrying electrical signals and responding to electrical stimuli. Two examples of electrical stimulation may include magnetic or inductive stimulation which may make use of a changing magnetic field, and electric or capacitive stimulation in which an electric field may be applied to the tissue.
For example, when an electrical conductor is passed through a magnetic field, an electric field is induced causing current to flow in the conductor. Because various parts of the body are also conductive, when a changing magnetic field is applied to the portion of the body, an electric field is created in the conductive tissue, thereby causing current to flow. In the context of biological tissue, for example, the resultant flow of electric current can stimulate the tissue by causing neurons in the tissue to depolarize. Also, in the context of muscles, for example, muscles associated with the stimulated neurons contract.
Electrical stimulation has many beneficial and therapeutic biological effects. For example, magnetic stimulation is effective in rehabilitating injured or paralyzed muscle groups. Magnetic stimulation is also proving effective for treatment of the spine, which is difficult to access directly because vertebrae surround it. Magnetic stimulation may be used to block the transmission of pain via nerves in the back (e.g., the nerves responsible for lower back pain) or via other nerves in other locations. Further, and unlike the other medical processes that stimulate the body, magnetic field stimulation may be non-invasive. For example, using magnetic fields to generate current in the body produces stimulation by passing the magnetic field through the skin of a patient. In contrast, conventional electric stimulation pain treatment methods for lower back pain involve the placement of electrodes in the spinal cord to enable stimulation. For peripheral nerves, a needle may be inserted proximate the problematic nerve to enable electric stimulation.
Magnetic stimulation also has proven effective in stimulating regions of the brain. One area of particular therapeutic interest is the treatment of neuropsychiatric disorders. It is believed that more than 28 million people in the United States alone suffer from some type of neuropsychiatric disorder. These include specific conditions such as depression, schizophrenia, mania, obsessive-compulsive disorder, panic disorders, just to name a few. One particular condition, depression, is believed to affect 19 million people in the United States alone, and possibly 340 million people worldwide. Modern medicine offers depression patients a number of treatment options, including several classes of anti-depressant medications such as selective serotonin reuptake inhibitors (SSRIs), MAIs, tricyclics, lithium, and electroconvulsive therapy (ECT). Yet many patients remain without satisfactory relief from the symptoms of depression. To date, ECT remains an effective treatment for major depressive disorder; however, many patients will not undergo the procedure because of its severe side effects.
Recently, repetitive transcranial magnetic stimulation (rTMS) has been shown to have significant anti-depressant effects for patients, even those that do not respond to the traditional methods and medications. In one embodiment of rTMS, a subconvulsive stimulation is applied to the prefrontal cortex in a repetitive manner, causing a depolarization of cortical neurons. The neurons are depolarized by the induction of an electric field, usually in excess of one volt per centimeter (V/cm). These electric fields result from a rapidly changing magnetic field applied non-invasively.
It is now well known to those skilled in the art that both the left and right prefrontal cortex regions of the brain have strong communication links to Limbic System structures, which contain the “circuits” controlling mood and general behavior. One objective of rTMS is to provide stimulation to these biological circuits through a non-invasive, sub-convulsive technique to relieve the symptoms of depression without many of the negative side effects of ECT or medications. However, one reported side effect of rTMS for the treatment of depression is patient discomfort at the site of the stimulation. It should be appreciated that discomfort may also be present in other forms of magnetic stimulation to other areas of the body. In the case of rTMS, this discomfort is caused, in part, by the depolarization of neuron membranes in the scalp and the resulting scalp muscle contractions that occur at the frequency of the rTMS. Testing has shown that approximately 20% of rTMS patients report this discomfort to be at a level that is very uncomfortable. In general, the greater the power and the higher the frequency of the therapeutic magnetic stimulation, the more discomfort is reported. Yet, reducing the power levels may not be a viable option because greater power has been shown to desirably stimulate deeper structures. Also, relatively higher frequencies (e.g., greater than 1 Hertz (Hz)) have been shown to have a greater anti-depressant effect.
Other types of discomfort may be caused by any number of ailments, and not exclusively as a side effect of rTMS. In addition, a patient may experience discomfort at locations other than the head area, such as at peripheral nerves and the like. Such locations may be at any location and at any depth within a patient's body. In such cases, it would be advantageous to reduce discomfort regardless of where the discomfort-causing nerve is located. Accordingly, it is desirable to develop techniques for reducing discomfort, whether caused by electrical stimulation or by other causes. In addition, it is desirable to develop techniques to more accurately stimulate a desired region of a patient while minimizing stimulation of surrounding tissue.